1. Field of the Invention
The present invention relates to a method of and apparatus for removing non-metallic inclusions and inter-metallic compounds from a molten metal.
2. Description of the Related Art
Hitherto, it has been known that presence of non-metallic inclusions and inter-metallic compounds (collectively referred to as "inclusions") causes defects in the metal products and hampers production of the metal products, and studies and proposals have been made for techniques for efficiently removing such inclusions.
For instance, it has been proposed to remove inclusions by allowing the molten metal to pass through a vessel interposed between a ladle and a mold. This method, however, introduces a problem in that the inclusions become involved in a high-velocity flow of the molten metal from the vessel (referred to as "short-circuit" or "short-pass") and are undesirably discharged together with the molten metal. In order to overcome this problem, it has been proposed to employ a vessel of a large volume and to provide a plurality of stages of dams which might be effective in preventing the above-mentioned short-circuit, thereby to avoid undesirable discharge of the inclusions. The provision of the dams, however, does not provide appreciable solution to the problem and involves more expense due to an increase in the quantity of the refractories to be used and due to a higher installation cost.
In this field of concern, Japanese Patent Unexamined Publications having Nos. 55-107743 and 58-22317, disclose a method in which horizontal rotational flow of molten metal concentrates the impurities into the central region by centrifugal effect, thus separating inclusions.
Even with this method, however, as described later, it is hard to prevent the discharge of inclusions when the mean residence time of the molten metal in the tundish is less than 40 seconds.
It is also noted that a mechanical separation method such as centrifugation is effective only in separating comparatively large particles of impurities and is not efficient in separation of small particles.
In contrast, Japanese Patent Unexamined Publication No. 61-103654 discloses a method in which generation of inclusions and removal of small particles of impurities are attained by heating the molten metal. This method, however, has no appreciable effect in preventing the introduction of large particles of inclusions, particularly slag, a phenomenon caused by the lowered level of the molten metal in the vessel in the intermittent condition when the vessel is waiting for the start of pouring of molten metal into the vessel from a next ladle.